Question

A common prank on college campuses is to switch the salt and sugar on dining hall tables, which is usually easy because the substances look so much alike. Yet, despite the similarity in their appearance, these two substances differ greatly in their properties, since one is a molecular solid and the other is an ionic solid. How do the properties differ and why?

Short answer

Sugar, a molecular solid, and salt, an ionic solid, have different properties due to the forces holding their structures together. Molecular solids have a lattice structure held together by weaker intermolecular forces (e.g., hydrogen bonds, van der Waals forces), while ionic solids have a crystal lattice held together by strong electrostatic forces. As a result, molecular solids generally have lower melting points, are soluble in water through hydrogen bonding, and are poor conductors of electricity. In contrast, ionic solids have higher melting points, dissolve in water through ion-water interactions, and can conduct electricity when dissolved or melted.

Step-by-step solution

Identify the Molecular and Ionic Solids

In this case, sugar is a molecular solid, and salt is an ionic solid. Molecular solids are made up of molecules held together by intermolecular forces, while ionic solids are made up of positively and negatively charged ions held together by electrostatic forces.

Describe the Structure of Molecular and Ionic Solids

Molecular solids, such as sugar, have a lattice structure formed by molecules that are held together by intermolecular forces (e.g., hydrogen bonds, van der Waals forces). These forces are relatively weaker compared to the electrostatic forces in ionic solids. Ionic solids, such as salt, have a lattice structure formed by ions. The positively charged ions (cations) and negatively charged ions (anions) are held together by strong electrostatic forces (ionic bonds) in a crystal lattice.

Compare Melting Points

Molecular solids generally have lower melting points compared to ionic solids. This is because the intermolecular forces holding the molecules together are weaker than the electrostatic forces holding the ions together in ionic solids. In the case of sugar and salt, sugar (a molecular solid) has a lower melting point than salt (an ionic solid).

Compare Solubility in Water

Both molecular and ionic solids can be soluble in water. However, their solubility mechanisms differ. In molecular solids like sugar, the molecules dissolve in water due to the formation of hydrogen bonds between the water molecules and the sugar molecules. In ionic solids like salt, the ions are attracted to and surrounded by water molecules, which can then separate the positive and negative ions and dissolve the salt. The electrostatic forces between the ions are overcome by the attractive forces between the ions and water molecules.

Compare Conductivity

Molecular solids are generally poor conductors of electricity because they do not have freely moving charged particles. In sugar, for instance, there are no free ions or electrons to carry an electric charge. On the other hand, ionic solids, such as salt, can conduct electricity when dissolved in water or melted. In these cases, the ions are free to move and can carry the electric charge through the solution or the melted solid. In conclusion, sugar (a molecular solid) and salt (an ionic solid) have different properties due to the types of bonding and forces that hold their structures together. Their structures, melting points, solubilities, and conductivities differ, resulting in distinct characteristics for each substance.